This invention relates to liquid cryogen pumps and, more particularly, to an improved pump for compressing, subcooling and transferring liquid helium.
Cryogenic liquids which are easily vaporized at atmospheric temperatures are difficult to handle because of the need to maintain them below their boiling points. Liquid helium is particularly difficult to handle because of its extremely low boiling point of about 4.2.degree. K. at one atmosphere, with a critical temperature of only 5.2.degree. K. In addition, it requires far less heat to vaporize helium than other liquid cryogens.
When pumping a liquid cryogen, a certain amount of heat will inevitably be introduced to the liquid from pump friction and transfer from the atmosphere. For this reason, cryogens are sometimes subcooled prior to pumping by heat transfer from colder fluids. Because liquid helium has such a low boiling point, it cannot be economically subcooled very much prior to pumping so that heat added to the liquid helium during pumping will readily cause vaporization resulting in low pump efficiency.
Conventional reciprocating piston-cylinder type pumps are not well suited for pumping liquid helium because the heat generated by friction causes vaporization of the helium. This is because such pumps are generally designed to minimize leakage around the piston, with the result that heat is generated by the friction between the piston and the cylinder. U.S. Pat. No. 2,054,710 issued to J. Okada on Sept. 15, 1936 relates to such a pump constructed so that a substantial amount of the fluid in the variable volume pumping chamber is returned to the supply through holes provided for that purpose in the cylinder wall which are located so as to extend from about 80% of the suction stroke to the end thereof. Such an arrangement results in an unduly high loss of efficiency. U.S. Pat. No. 3,431,744 issued to R. Veilex et al on Mar. 11, 1969 relates to a reciprocating piston-cylinder type pump not suited for pumping a liquid such as helium which is not desired to be vaporized. In the Veilex pump, the pump discharge is through the piston to the space above and no provision is made for removing vapor from the pump discharge. Other types of pumps, such as centrifugal pumps, have been used with some success, but generally only for relatively low pressure differentials below about 7 psi (48 kPa) and high flow rates above about 10 gal/min (6.3.times.10.sup.4 m.sup.3 /sec).